How the New EU Wastewater Directive Changes Existing Plants

Introduction

Directive (EU) 2024/3019, the recast Urban Wastewater Treatment Directive, changes the long-term framework under which existing wastewater treatment plants will be assessed and upgraded across the EU. For plant owners and operators, the practical issue is not only the legal deadline, but the fact that secondary biological treatment remains the baseline expectation for a much broader set of agglomerations.

The directive entered into force on 1 January 2025. It repeals the old Directive 91/271/EEC with effect from 1 August 2027, and Member States must transpose the new rules into national law by 31 July 2027. National implementation will vary, but the EU-wide direction is clear: more plants will need reliable biological treatment, better effluent quality, and a clearer pathway to further upgrades.

For a summary of the EU framework, see /eu-directive.

What changes for existing treatment plants

The new directive does not simply redefine future builds; it also affects existing works that are already in operation. The main change is the widening and tightening of the secondary-treatment baseline.

Secondary treatment becomes the core expectation

Secondary treatment is defined as treatment that generally involves biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter. In practice, this means existing plants that rely on primary treatment alone, lagooning without equivalent biological performance, or other limited processes will need to be reviewed against the new requirements.

Secondary treatment is mandatory for agglomerations of 2,000 p.e. and above. That obligation is extended to agglomerations of 1,000 p.e. and above by 31 December 2035, subject to transitional and exemption arrangements, including for certain coastal areas and for Bulgaria, Croatia and Romania.

For existing plants, the implication is straightforward:

• some smaller works will move into scope for the first time;
• some plants already meeting the old rules may still need process optimisation or physical upgrades;
• operators should verify whether the current works can consistently meet the relevant secondary effluent standard.

The standard is not only legal, but operational

Annex I sets the secondary-treatment effluent standards at:

• BOD5: 25 mg/L, or 70-90% reduction;
• COD: 125 mg/L, or 75% reduction;
• TSS: 35 mg/L.

These are not just numbers on a permit. They are a signal that existing plants must maintain stable biological performance, reliable clarification, and adequate solids capture under normal and peak loading conditions.

What this means in process terms

Most existing plants will not need a completely new treatment concept. However, many will need some combination of hydraulic, biological, and solids-handling improvements to ensure compliance over time.

Biological capacity and process stability

For activated sludge and other suspended-growth systems, compliance depends on the ability to remove biodegradable organic matter consistently. Common design values such as sludge age, dissolved oxygen setpoints, MLSS concentration, and F/M ratio are site-specific and should be checked against actual influent load, temperature, storm ingress, industrial contribution, and seasonal variation.

Indicative questions for existing plants include:

• Is the aeration system delivering enough oxygen under peak BOD loading?
• Is the biological stage robust enough for wet-weather dilution and shock loads?
• Is there adequate process control for ammonia and carbon removal where nitrification or tighter performance is already required?
• Is the aeration basin volume still appropriate after population growth or load drift?

A plant that meets limits on paper but fails during wet weather, low temperature, or sludge bulking conditions may still be at risk once compliance is reviewed more closely.

Secondary settlement and solids separation

Because the directive defines secondary treatment as generally including a secondary settlement stage, clarifier performance becomes central to compliance. Existing settlement tanks may need review for:

• surface overflow rate under peak flow;
• sludge blanket control;
• return activated sludge capacity;
• scum removal and hydraulic distribution;
• solids carryover during storm events.

Where final effluent TSS is close to the limit, even modest process disturbances can push the plant out of compliance. In many cases the issue is not the biology alone, but the interface between biology and clarification.

Older assets often need targeted rehabilitation

Existing plants frequently have strong core civil assets but outdated mechanical and electrical equipment. Typical upgrade areas include:

• coarse and fine screening;
• aeration equipment and blowers;
• final settlement tank drives and scraper mechanisms;
• internal recycle and sludge return pumping;
• instrumentation and control;
• standby power and resilience measures.

These works are often less disruptive than a full plant replacement and can deliver the process stability needed to maintain secondary-treatment performance.

Future upgrade pressure beyond secondary treatment

Although the focus here is secondary treatment, existing plants should not look at the directive in isolation. The same recast framework also introduces future obligations for nutrient removal, micropollutant removal and energy performance.

Tertiary treatment will affect larger plants and sensitive receiving waters

Tertiary treatment for nitrogen and phosphorus removal is required for plants of 150,000 p.e. and above by 2039, and for plants of 10,000 p.e. and above in areas at risk of eutrophication by 2045.

For existing plants, this matters because decisions made now about aeration, clarifier headroom, sludge handling and process layout can either support or constrain future nutrient upgrades. A plant that is only just adequate for carbon removal may face a much more difficult retrofit later.

Quaternary treatment and energy neutrality are part of the long-term trajectory

Quaternary treatment for micropollutant removal is required for plants of 150,000 p.e. and above by 2045, with interim milestones in 2033 and 2039. The directive also sets an EU-wide objective for energy neutrality in the urban wastewater treatment sector by 2045.

For existing plants, this means that upgrade planning should not be limited to immediate effluent compliance. It should also consider:

• process energy demand;
• opportunities for digester gas use where applicable;
• pumping and aeration efficiency;
• sidestream management;
• future space requirements for tertiary or quaternary stages.

Practical implications for plant owners and operators

The biggest change for existing plants is not a single mandatory retrofit date for every works. It is the need to establish a credible compliance pathway.

1) Check the current status against the new scope

Start by confirming:

• agglomeration size in p.e.;
• whether the plant is within secondary-treatment scope now or by 31 December 2035;
• whether any transitional or exemption provisions may apply;
• whether the receiving water or future local plan may trigger nutrient removal later.

Population equivalent is defined as the organic biodegradable load having a five-day biochemical oxygen demand of 60 g of oxygen per day. That definition matters when comparing design load, actual loading and growth forecasts.

2) Review actual performance, not just design intent

Many plants were designed for different influent characteristics, hydraulics or discharge expectations. A useful review should consider:

• recent effluent BOD5, COD and TSS performance;
• wet-weather performance;
• peak flow bypass risk;
• sludge age and biomass condition;
• clarifier upsets and solids washout;
• maintenance backlog and asset condition.

3) Build an upgrade sequence

For most existing plants, the sensible sequence is:

• stabilise the current secondary process;
• remove bottlenecks in aeration or clarification;
• improve control and monitoring;
• confirm headroom for future loading;
• reserve space and interfaces for later nutrient or micropollutant stages where relevant.

This approach reduces the risk of spending capital on a short-term fix that later has to be undone.

4) Align compliance planning with asset management

The directive should be treated as both a regulatory and an asset-planning issue. A plant with ageing blowers, underperforming clarifiers or fragile instrumentation is already exposed. The directive simply makes that exposure more visible and more consequential.

How this applies to your plant

If your plant is an existing secondary-treatment works, the key question is whether it can reliably meet Annex I standards today and remain compliant as the scope expands. If it is below 2,000 p.e., review whether it may come into scope later. If it is 2,000 p.e. or above, confirm that the biological stage, secondary settlement and solids handling are robust enough for continuous compliance. If it is larger or discharges to a sensitive receiving water, start planning for future nutrient and energy requirements now.

The most practical next step is a structured engineering review of process capacity, hydraulic bottlenecks and upgrade options. You can Start the Engineering Assessment to evaluate the current treatment train and identify the most appropriate secondary-treatment improvements.